The present invention generally relates to a method and system for inerting aircraft fuel tanks by providing a nitrogen enriched air supply.
It is recognized that fuel vapors within fuel tanks create become combustible in the presence of oxygen. An inerting system decreases the probability of combustion of inflammable materials stored in a fuel tank by maintaining a chemically non-reactive or inert gas, such as nitrogen, in the fuel tank vapor space also known as ullage. Three elements are required to initiate and sustain combustion: an ignition source (e.g., heat), fuel, and oxygen. Combustion may be prevented by reducing any one of these three elements. If the presence of an ignition source cannot be prevented within a fuel tank, then the tank may be made inert by: 1) reducing the oxygen concentration threshold, 2) reducing the fuel concentration of the ullage to below the lower explosive limit (LEL), or 3) increasing the fuel concentration to above the upper explosive limit (UEL). Many systems reduce the risk of combustion by reducing the oxygen concentration by introducing an inert gas such as nitrogen to the ullage, thereby displacing oxygen with nitrogen.
It is known in the art to equip aircraft with onboard inert gas generating systems, which supplies nitrogen-enriched air (NEA) to the vapor space (i.e., ullage) within the fuel tank. The nitrogen-enriched air has a substantially reduced oxygen content that reduces or eliminates combustible conditions within the fuel tank. Onboard inert gas generating systems typically use membrane-based gas separators. Such separators contain a membrane that is permeable to oxygen molecules, but relatively impermeable to nitrogen molecules. A pressure differential across the membrane causes oxygen molecules from air on one side of the membrane to pass through the membrane, which forms oxygen-enriched air (OEA) on the low-pressure side of the membrane and NEA on the high-pressure side of the membrane. The requirement for a pressure differential necessitates a source of compressed or pressurized air. Bleed air from an aircraft engine or from an onboard auxiliary power unit can provide a source of compressed air; however, this can reduce available engine power and also must compete with other onboard demands for compressed air, such as the onboard air environmental conditioning system. Moreover, certain flight conditions such as during aircraft descent can lead to an increased demand for NEA at precisely the time when engines are throttled back so that sufficient compressed air may not be available. Additionally, there is a trend to reduce or eliminate bleed-air systems in aircraft; for example Boeing's 787 has a no-bleed systems architecture, which utilizes electrical systems to replace most of the pneumatic systems in order to improve fuel efficiency, as well as reduce weight and lifecycle costs. A separate compressor or compressors can be used to provide pressurized air to the membrane gas separator, but this undesirably increases aircraft payload, and also represents another onboard device with moving parts that are subject to maintenance issues or device failure. Additionally, the membranes in such gas separators are subject to fouling over time.